1. Field of the Invention
The present invention relates to a technique for the microbial industry, more precisely, a method for producing a basic substance by fermentation. As basic substances which are able to be produced by fermentation, for example, L-lysine is useful as an additive in animal feed, and L-arginine and L-histidine are useful for pharmaceutical preparations such as infusions.
2. Brief Description of the Related Art
In the methods for producing basic substances by fermentation, microorganisms having an ability to produce a basic substance are cultured to produce and accumulate the basic substance in a medium, and the basic substance is collected from the medium. In such methods, the culture is performed as batch culture, feeding culture or continuous culture.
In such production of basic substances by fermentation, sulfate or chloride ions have been typically added to a medium as counter anions for an objective substance which dissociates into a cation in the medium in order to maintain pH of the medium at a neutral level (Japanese Patent Laid-open (Kokai) Nos. 5-30985 and 5-244969).
In many cases, basic substances are collected from a medium by ion exchange, when purification is required. For example, in the case of L-lysine, after fermentation broth is made weakly acidic, L-lysine is adsorbed on an ion exchange resin and then eluted from the resin with ammonium ions. The eluted L-lysine is used as it is as lysine base, or it can be crystallized with hydrochloric acid to form L-lysine hydrochloride.
When chloride ions are used as counter anions in the medium in the aforementioned purification of L-lysine, L-lysine hydrochloride can be obtained directly by concentrating the medium. However, since chloride ions corrode metal fermentation tanks etc., it is not preferable to make them exist in the medium in high concentration in actual production.
On the other hand, when the basic substance is not purified, the fermentation broth is concentrated as it is, or it is made weakly acidic with hydrochloric acid or sulfuric acid, followed by spray granulation. In this case, the residual components contain the counter anions added to the medium, and therefore the amount of the basic substance is reduced in the resulting fermentation product.
Japanese Patent Laid-open No. 2002-65287 (U.S. Patent Application No. 2002025564) discloses a method of utilizing, in the production of a basic amino acid by fermentation, carbonate and bicarbonate ions as counter anions of the basic amino acid to substitute for a part of sulfate or chloride ions. Carbonate and bicarbonate ions can be comparatively easily removed from the culture medium by making the pH of the medium acidic, or concentrating the medium, or both. The above-cited publication teaches a method of controlling the internal pressure in the fermentation tank so that it is positive during fermentation, or adding carbon dioxide gas or a mixed gas containing carbon dioxide to the medium, as a means for adding carbonate ions and bicarbonate ions to the medium. However, at typical medium conditions, such as a neutral pH, only a small amount of carbon dioxide gas dissolves, if at all. Therefore, to maintain the presence of bicarbonate and carbonate ions in the culture medium so that the effect of reducing the sulfate or chloride ion concentration is maintained, the culture must be performed at an alkaline pH. However, if pH becomes high, the bacterial growth rate and productivity of the objective substance are generally reduced.